Features

• Higher Detection Capability and Better Linear Response in the IR Than Typical PIN Junction Photodiodes.
• Detects Light Signals From 0.2 - 1 kHz (PDA30G) or 0.2 - 10 kHz (PDA20H)
• Two 8-32 (M4) Tapped Mounting Holes

Thorlabs offers two types of amplified photoconductor detectors, Lead Sulfide (PbS) and Lead Selenide (PbSe). Photoconductors are widely used for the detection of mid-IR (MIR) radiation from 1.0 to 4.8 µm. They detect light in a broader wavelength range, offer higher detection capability, and provide better linear response in the IR than typical PIN junction photodiodes. The slim profile housing enables use in light paths with space constraints. All connections and controls are located perpendicular to the light path, providing increased accessibility. Amplification is provided by low noise transimpedance or voltage amplifiers that are capable of driving 50 Ω loads. Signal output is via a BNC connector. Thorlabs offers a wide variety of BNC, BNC-to-SMA, and SMC cables, as well as a variety of BNC, SMA, and SMC adapters.

Each housing provides two 8-32 tapped mounting holes (M4 for - EC) centered on the detector surface for vertical or horizontal post mounting. The housings also feature external SM1 threading and internal SM05 threading that are compatible with most Thorlabs SM1 (1.035"-40)- and SM05 (0.535"-40)-threaded accessories. Additionally, an internally threaded SM1 coupler is included with each detector. This allows convenient mounting of SM1 compatible accessories, optics, and cage assembly accessories. The internal SM05 threading is only suitable for mating to an externally threaded SM05 lens tube (components such as fiber adapters cannot be threaded onto the SM05 threading). SM1-threaded fiber adapters may be used with any of these detectors. Externally SM1-threaded adapters should be mated to the included internally SM1-threaded adapter, while internally SM1-threaded adapters can be mated directly to the housing. A 120 VAC AC/DC linear power supply is included (230 VAC for - EC versions).

Due to limitations in the IC, the high-speed amplifier used in these devices may become unstable, exhibiting oscillations or negative output if the linear power supply voltage is applied when the module is on. The unit should always be powered up using the power switch on the power supply or the unit itself. Hot plugging the unit is not recommended. Additionally, inhomogeneities at the edges of the active area of the detector can generate unwanted capacitance and resistance effects that distort the time-domain response of the photodetector output. Thorlabs therefore recommends that the incident light on the photodetector is well centered on the active area. The SM1 (1.035"-40) threading on the housing is ideally suited for mounting a Ø1" focusing lens or pinhole in front of the detector element.

Photoconductors vs. Photodiodes
Unlike PIN junction photodiodes, which generate a photocurrent when light is absorbed in the depleted region of the junction semiconductor, the photoconductive material in these devices exhibits a decrease in electrical resistance when illuminated with IR radiation. Photoconductive detectors typically have a very linear response when illuminated with IR radiation.

Usage Notes
Photoconductors function differently than typical PIN junction photodiodes. We recommend that an optical chopper be employed when using these detectors with CW light, due to signal noise issues. PbS and PbSe detectors can be used at room temperature. However, temperature fluctuations will affect dark resistance, sensitivity, and response speeds.

If greater detector bandwidth is desired, Thorlabs manufactures HgCdTe and InAsSb detectors for the mid-IR that offer 160 kHz and 1600 kHz bandwidth, respectively.

Performance Specifications

^a A NEP range is given for the switch gain detectors, a max NEP is given for the fixed gain detectors.
^b Please note that rise times depend on the chosen gain level. As one increases the gain of a given optical amplifier, the bandwidth is reduced, and hence, the rise time increases.
^c This detector has an AC coupled amplifer.

Gain Specifications 

Note: Gain figures can also be expressed in units of Ω.

PDA Series Design, scale in inches [mm ].

Compact PDA & PDF Series Design

Thorlabs' Amplified Photodiode series features a slim design, which allows the detector access to the light path even between closely spaced optical elements.

The power supply input and the BNC output are located on the same outer edge of the package, further reducing the device thickness and allowing easier integration into tight optic arrangements. The PDA and PDF series detectors can fit into spaces as thin as 0.83" (21.1 mm) when the SM1 coupler is removed. With the SM1 coupler attached, the smallest width the detector can fit into is 1.03" (26.2 mm).

Additionally, the detectors have two tapped mounting holes perpendicular to each other so that the unit can be mounted in a horizontal or vertical orientation. This dual mounting feature offsets the fact that the cables protrude out the side of the package, thus requiring more free space above or alongside your beam path.

The switchable gain detectors feature an eight-position rotary gain switch (pictured below right) mounted on an outside edge perpendicular to the power supply and BNC output connections. The location of the gain switch allows for easy adjustments while the detector is mounted.

PDA Series Mounting Options

The PDA series of amplified photodetectors are compatible with our entire line of lens tubes, TR series posts, and cage mounting systems. Because of the wide range of mounting options, the best method for mounting the housing in a given optical setup is not always obvious. The pictures and text in this tab will discuss some of the common mounting solutions. As always, our technical support staff is available for individual consultation.

Picture of a PDA series photodetector as it will look when unpackaged.	Picture of a DET series photodetector with the included SM1T1 and its retaining ring removed from the front of the housing. Thorlabs' PDA series photodetectors feature the same mounting options.	A close up picture of the front of the PDA10A photodetector. The internal SM1 threading on the SM1T1 adapter and internal SM05 threading on the photodetector housing can be seen in this image.

TR Series Post (Ø1/2" Posts) System

The PDA housing can be mounted vertically or horizontally on a TR Series Post using the 8-32 (M4) threaded holes.

DET series photodetector mounted vertically on a TR series post. In this configuration, the output and power cables (PDA series) are oriented vertically and away from the optic table, facilitating a neater optical setup.	PDA series photodetector mounted horizontally on a TR series post. In this configuration, the on/off switch is conveniently oriented on the top of the detector.

Lens Tube System

Each PDA housing includes a detachable Ø1" Optic Mount (SM1T1) that allows for Ø1" (Ø25.4 mm) optical components, such as optical filters and lenses, to be mounted along the axis perpendicular to the center of the photosensitive region. The maximum thickness of an optic that can be mounted in the SM1T1 is 0.1" (2.8 mm). For thicker Ø1" (Ø25.4 mm) optics or for any thickness of Ø0.5" (Ø12.7 mm) optics, remove the SM1T1 from the front of the detector and place (must be purchased separately) an SM1 or SM05 series lens tube, respectively, on the front of the detector.

The SM1 and SM05 threadings on the PDA photodetector housing make it compatible with our SM lens tube system and accessories. Two particularly useful accessories include the SM-threaded irises and the SM-compatible IR and visible alignment tools. Also available are fiber optic adapters for use with connectorized fibers.

DET series photodetector mounted onto an SM1L30C Ø1" Slotted Lens Tube, which is housing a focusing optic. The lens tube is attached to a 30 mm cage system via a CP02 SM1-Threaded 30 mm Cage Plate. This arrangement allows easy access for optic adjustment and signal alignment.

Cage System

The simplest method for attaching the PDA photodetector housing to a cage plate is to remove the SM1T1 that is attached to the front of the PDA when it is shipped. This will expose external SM1 threading that is deep enough to thread the photodetector directly to a CP02 30 mm cage plate. When the CP02 cage plate is tightened down onto the PDA photodetector housing, the cage plate will not necessarily be square with the detector. To fix this, back off the cage plate until it is square with the photodetector and then use the retaining ring included with the SM1T1 to lock the PDA photodetector into the desired location.

This method for attaching the PDA photodetector housing to a cage plate does not allow much freedom in determining the orientation of the photodetector; however, it has the benefit of not needing an adapter piece, and it allows the diode to be as close as possible to the cage plate, which can be important in setups where the light is divergent. As a side note, Thorlabs sells the SM05PD and SM1PD series of photodiodes that can be threaded into a cage plate so that the diode is flush with the front surface of the cage plate; however, the photodiode is unbiased.

For more freedom in choosing the orientation of the PDA photodetector housing when attaching it, a SM1T2 lens tube coupler can be purchased. In this configuration the SM1T1 is left on the detector and the SM1T2 is threaded into it. The exposed external SM1 threading is now deep enough to secure the detector to a CP02 cage plate in any orientation and lock it into place using one of the two locking rings on the ST1T2.

This picture shows a DET series photodetector attached to a CP02 cage plate after removing the SM1T1. The retaining ring from the SM1T1 was used to make the orientation of the detector square with the cage plate.	These two pictures show a DET series photodetector in a horizontal configuration. The top picture shows the detector directely coupled to a CP02 cage plate. The bottom picture shows a DET series photodetector attached to a CP02 cage plate using an SM1T2 adapter in addition to the SM1T1 that comes with the PDA series detector.

Although not pictured here, the PDA photodetector housing can be connected to a 16 mm cage system by purchasing an SM05T2. It can be used to connect the PDA photodetector housing to an SP02 cage plate.

Application

The image below shows a Michelson Interferometer built entirely from parts available from Thorlabs. This application demonstrates the ease with which an optical system can be constructed using our lens tube, TR series post, and cage systems. A PDA series photodetector is interchangable with the DET series photodetector shown in the picture.

The table below contains a part list for the Michelson Interferometer for use in the visible range. Follow the links to the pages for more information about the individual parts. 

The following table lists the photodiodes found on this page, along with the mounted photodiodes and packaged detectors which use the same internal photodiode.

PbS and PbSe Photoconductive Detectors

Lead Sulfide (PbS) and Lead Selenide (PbSe) photoconductive detectors are widely used in detection of infrared radiation from 1000 to 4800 nm. Unlike standard photodiodes, which produce a current when exposed to light, the electrical resistance of the photoconductive material is reduced when illuminated with light. Although PbS and PbSe detectors can be used at room temperature, temperature flucturations will affect dark resistance, sensitivity, and response speeds (see Temperature Considerations below).

Photoconductor Basic Model

Click to Enlarge

Theory of Operation

For photoconductive materials, incident light will cause the number of charge carriers in the active area to increase, thus decreasing the resistance of the detector. This change in resistance leads to a change in measured voltage, and hence, photosensitivity is expressed in units of V/W. An example operating circuit is shown to the right. Please note that the circuit depicted is not recommended for practical purposes since low frequency noise will be present.

The detection mechanism is based upon the conductivity of the thin film of the active area. The output signal of the detector with no incident light is defined by the following equation:

A change ΔV_OUT then occurs due to a change ΔR_Dark in the resistance of the detector when light strikes the active area:

Frequency Response
Photoconductors must be used with a pulsed signal to obtain AC signals. Hence, an optical chopper should be employed when using these detectors with CW light. The detector responsivity (R_f) when using a chopper can be calculated using the equation below:

Here, f_c is the chopping frequency, R₀ is the response at 0 Hz, and τ_r is the detector rise time.

Effects of Chopping Frequency
The photoconductor signal will remain constant up to the time constant response limit. PbS and PbSe detectors have a typical 1/f noise spectrum (i.e., the noise decreases as chopping frequency increases), which has a profound impact on the time constant at lower frequencies.

The detector will exhibit lower responsivity at lower chopping frequencies. Frequency response and detectivity are maximized for

The characteristic curve for Signal vs. Chopping Frequency for each particular detector is provided in chapter 4 of the operating manuals.

Temperature Considerations
These detectors consist of a thin film on a glass substrate. The effective shape and active area of the photoconductive surface varies considerably based upon the operating conditions, thus changing performance characteristics. Specifically, responsivity of the detector will change based upon the operating temperature.

Temperature characteristics of PbS and PbSe bandgaps have a negative coefficient, so cooling the detector shifts its spectral response range to longer wavelengths. For best results, operate the photodiode in a stable controlled environment. See the Operating Manuals for characteristic curves of Temperature vs. Sensitivity for a particular detector.

Typical Photoconductor Amplifier Circuit

Due to the noise characteristic of a photoconductor, it is generally suited for AC coupled operation. The DC noise present with the applied bias will be too great at high bias levels, thus limiting the practicality of the detector. For this reason, IR detectors are normally AC coupled to limit the noise. A pre-amplifier is required to help maintain the stability and provide a large gain for the generated current signal.

Based on the schematic below, the op-amp will try to maintain point A to the input at B via the use of feedback. The difference between the two input voltages is amplified and provided at the output. It is also important to note the high pass filter that AC couples the input of the amplifier blocks any DC signal. In addition, the resistance of the load resistor (R_LOAD) should be equal to the dark resistance of the detector to ensure maximum signal can be acquired. The supply voltage (+V) should be at a level where the SNR is acceptable and near unity. Some applications require higher voltage levels; as a result the noise will increase. Provided in chapter 4 of the Operating Manual is a SNR vs. Supply Voltage characteristic curve to help determine best operating condition. The output voltage is derived as the following:

Amplifier Model

Signal to Noise Ratio
Since the detector noise is inversely proportional to the chopping frequency, the noise will be greater at low frequencies. The detector output signal is linear to increased bias voltage, but the noise shows little dependence on the bias at low levels. When a set bias voltage is reached, the detector noise will increase linearly with applied voltage. At high voltage levels, noise tends to increase exponentially, thus degrading the signal to noise ratio (SNR) further. To yield the best SNR, adjust the chopping frequency and bias voltage to an acceptable level. Provided in chapter 4 of the operating manuals are characteristic curves for SNR vs. Chopping Frequency and SNR vs. Supply Voltage for each particular detector.

Noise Equivalent Power
The noise equivalent power (NEP) is the generated RMS signal voltage generated when the signal to noise ratio is equal to one. This is useful, as the NEP determines the ability of the detector to detect low level light. In general, the NEP increases with the active area of the detector and is given by the following equation:

Here, S/N is the Signal to Noise Ratio, Δf is the Noise Bandwidth, and Incident Energy has units of W/cm².

Dark Resistance
Dark Resistance is the resistance of the detector under no illumination. It is important to note that dark resistance will increase or decrease with temperature. Cooling the device will increase the dark resistance. Provided in chapter 4 of the operating manuals is a Dark Resistance vs. Temperature characteristic graph for each particular detector.

Detectivity (D) and Specific Detectivity (D*)
Detectivity (D) is another criteria used to evaluate the performance of the photodetector. Detectivity is a measure of sensitivity and is the reciprocal of NEP.

Higher values of detectivity indicate higher sensitivity, making the detector more suitable for detecting low light signals. Detectivity varies with the wavelength of the incident photon.

NEP of a detector depends upon the active area of the detector, which in essence will also affect detectivity. This makes it hard to compare the intrinsic properties of two detectors. To remove the dependence, Specific Detectivity (D^*), which is not dependent on detector area, is used to evaluate the performance of the photodetector.